The building of automotive vehicle bodies usually involves the joining of complementary panels or the attachment of a panel to a frame structure. Vehicle doors and other closure members are often made of inner and outer panels. The panels are usually made of like materials, for example stamped steel sheet workpieces or stamped aluminum sheets, and joined by welding, hemming, mechanical fasteners, or adhesive bonding. Also, in the construction of some vehicle bodies, panels are made of polymeric composite moldings and adhesively bonded.
Structural adhesives are known for joining panels of like metal alloys and panels molded of fiber reinforced sheet molding compound for automotive applications. For example, commercial epoxy resins are suitable for this purpose. Urethane resins are also used in structural adhesive bonding applications. Adhesives for structural applications are dispensed in either one or two parts, where, in a two-part formulation; one of the parts contains the catalyst that hardens the adhesive. In a one part formulation, the catalytic curing agent is contained in the adhesive resin formulation. The adhesive is formulated as a mobile viscous material that can be withdrawn from a reservoir, separate streams mixed if a two-part adhesive is employed, and extruded as a bead or layer onto a surface using, for example, a programmed robotic applicator. The adhesive is applied to the bonding surface of at least one of like-material workpieces and the intended bonding surfaces of the workpieces are pressed together. The assembly is then typically treated (usually by heating) to activate the catalyzed polymerization reactions by which the adhesive is hardened or cured in a strong adherent bond between surfaces of the panels or other part combinations.
Now there is interest in attaching body members of different materials in order to reduce the weight of the full range of automotive vehicles used worldwide. For example, inner and outer door panels, lift gate panels, hoods and deck lids can be made of any combination of steel panels, aluminum panels, magnesium panels, carbon composite panels, or SMC panels to satisfy structural, weight, and appearance requirements. Similar uses of mixed materials may be employed to create structural subsystems or body frames involving panels and structural members of various shapes including castings and extrusions. But the use of such combinations of mixed materials presents issues of how the body members can be efficiently and effectively attached and how corrosion due to galvanic action between closely spaced different materials can be minimized.